The image of NGC891 below was one of my first shots with a Takahashi Mewlon 250 that I recently acquired. It was taken with an Apogee AP6Ep camera. This has a Kodak KAF1000 chip with a diameter of 1024 x 1025 pixels (24.6mm x 24.6mm). This is a single ten-minute exposure @ f/12 that was calibrated in MaxIm DL and adjusted (Curves, resizing) in Photoshop.
Although the image initally looks pretty good, close inspection of the edges shows some distorted stars. Below are enlargements of the top right and bottom right corners of the frame, respectively.
The flaring appears to radiate from the same point. Interestingly, the stars on the extreme right half of the image look pretty round. This could be due to the CCD chip not being directly in the center of the image circle. No?
I realize that the Dall-Kirkham design is known to have some coma, but I have high expectations of Takahashi scopes. I own an FS-102 so I know what pinpoint stars look like. So are these imperfections something that can be reduced by better collimation? Will I just have to crop my images??
By the way, if you are interested in seeing the finished image of NGC891, click here.